/*
   * lib/ts_bm.c		Boyer-Moore text search implementation
   *
   *		This program is free software; you can redistribute it and/or
   *		modify it under the terms of the GNU General Public License
   *		as published by the Free Software Foundation; either version
   *		2 of the License, or (at your option) any later version.
   *
   * Authors:	Pablo Neira Ayuso <pablo@eurodev.net>
   *
   * ==========================================================================
   * 
   *   Implements Boyer-Moore string matching algorithm:
   *
   *   [1] A Fast String Searching Algorithm, R.S. Boyer and Moore.
   *       Communications of the Association for Computing Machinery, 
   *       20(10), 1977, pp. 762-772.
   *       http://www.cs.utexas.edu/users/moore/publications/fstrpos.pdf
   *
   *   [2] Handbook of Exact String Matching Algorithms, Thierry Lecroq, 2004
   *       http://www-igm.univ-mlv.fr/~lecroq/string/string.pdf
   *
   *   Note: Since Boyer-Moore (BM) performs searches for matchings from right 
   *   to left, it's still possible that a matching could be spread over 
   *   multiple blocks, in that case this algorithm won't find any coincidence.
   *   
   *   If you're willing to ensure that such thing won't ever happen, use the
   *   Knuth-Pratt-Morris (KMP) implementation instead. In conclusion, choose 
   *   the proper string search algorithm depending on your setting. 
   *
   *   Say you're using the textsearch infrastructure for filtering, NIDS or 
   *   any similar security focused purpose, then go KMP. Otherwise, if you 
   *   really care about performance, say you're classifying packets to apply
   *   Quality of Service (QoS) policies, and you don't mind about possible
   *   matchings spread over multiple fragments, then go BM.
   */
  
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/types.h>
  #include <linux/string.h>
  #include <linux/ctype.h>
  #include <linux/textsearch.h>
  
  /* Alphabet size, use ASCII */
  #define ASIZE 256
  
  #if 0
  #define DEBUGP printk
  #else
  #define DEBUGP(args, format...)
  #endif
  
  struct ts_bm
  {
  	u8 *		pattern;
  	unsigned int	patlen;
  	unsigned int 	bad_shift[ASIZE];
  	unsigned int	good_shift[0];
  };
  
  static unsigned int bm_find(struct ts_config *conf, struct ts_state *state)
  {
  	struct ts_bm *bm = ts_config_priv(conf);
  	unsigned int i, text_len, consumed = state->offset;
  	const u8 *text;
  	int shift = bm->patlen - 1, bs;
  	const u8 icase = conf->flags & TS_IGNORECASE;
  
  	for (;;) {
  		text_len = conf->get_next_block(consumed, &text, conf, state);
  
  		if (unlikely(text_len == 0))
  			break;
  
  		while (shift < text_len) {
  			DEBUGP("Searching in position %d (%c)
  ", 
  				shift, text[shift]);
  			for (i = 0; i < bm->patlen; i++) 
  				if ((icase ? toupper(text[shift-i])
  				    : text[shift-i])
  					!= bm->pattern[bm->patlen-1-i])
  				     goto next;
  
  			/* London calling... */
  			DEBUGP("found!
  ");
  			return consumed += (shift-(bm->patlen-1));
  
  next:			bs = bm->bad_shift[text[shift-i]];
  
  			/* Now jumping to... */
  			shift = max_t(int, shift-i+bs, shift+bm->good_shift[i]);
  		}
  		consumed += text_len;
  	}
  
  	return UINT_MAX;
  }
  
  static int subpattern(u8 *pattern, int i, int j, int g)
  {
  	int x = i+g-1, y = j+g-1, ret = 0;
  
  	while(pattern[x--] == pattern[y--]) {
  		if (y < 0) {
  			ret = 1;
  			break;
  		}
  		if (--g == 0) {
  			ret = pattern[i-1] != pattern[j-1];
  			break;
  		}
  	}
  
  	return ret;
  }
  
  static void compute_prefix_tbl(struct ts_bm *bm, int flags)
  {
  	int i, j, g;
  
  	for (i = 0; i < ASIZE; i++)
  		bm->bad_shift[i] = bm->patlen;
  	for (i = 0; i < bm->patlen - 1; i++) {
  		bm->bad_shift[bm->pattern[i]] = bm->patlen - 1 - i;
  		if (flags & TS_IGNORECASE)
  			bm->bad_shift[tolower(bm->pattern[i])]
  			    = bm->patlen - 1 - i;
  	}
  
  	/* Compute the good shift array, used to match reocurrences 
  	 * of a subpattern */
  	bm->good_shift[0] = 1;
  	for (i = 1; i < bm->patlen; i++)
  		bm->good_shift[i] = bm->patlen;
          for (i = bm->patlen-1, g = 1; i > 0; g++, i--) {
  		for (j = i-1; j >= 1-g ; j--)
  			if (subpattern(bm->pattern, i, j, g)) {
  				bm->good_shift[g] = bm->patlen-j-g;
  				break;
  			}
  	}
  }
  
  static struct ts_config *bm_init(const void *pattern, unsigned int len,
  				 gfp_t gfp_mask, int flags)
  {
  	struct ts_config *conf;
  	struct ts_bm *bm;
  	int i;
  	unsigned int prefix_tbl_len = len * sizeof(unsigned int);
  	size_t priv_size = sizeof(*bm) + len + prefix_tbl_len;
  
  	conf = alloc_ts_config(priv_size, gfp_mask);
  	if (IS_ERR(conf))
  		return conf;
  
  	conf->flags = flags;
  	bm = ts_config_priv(conf);
  	bm->patlen = len;
  	bm->pattern = (u8 *) bm->good_shift + prefix_tbl_len;
  	if (flags & TS_IGNORECASE)
  		for (i = 0; i < len; i++)
  			bm->pattern[i] = toupper(((u8 *)pattern)[i]);
  	else
  		memcpy(bm->pattern, pattern, len);
  	compute_prefix_tbl(bm, flags);
  
  	return conf;
  }
  
  static void *bm_get_pattern(struct ts_config *conf)
  {
  	struct ts_bm *bm = ts_config_priv(conf);
  	return bm->pattern;
  }
  
  static unsigned int bm_get_pattern_len(struct ts_config *conf)
  {
  	struct ts_bm *bm = ts_config_priv(conf);
  	return bm->patlen;
  }
  
  static struct ts_ops bm_ops = {
  	.name		  = "bm",
  	.find		  = bm_find,
  	.init		  = bm_init,
  	.get_pattern	  = bm_get_pattern,
  	.get_pattern_len  = bm_get_pattern_len,
  	.owner		  = THIS_MODULE,
  	.list		  = LIST_HEAD_INIT(bm_ops.list)
  };
  
  static int __init init_bm(void)
  {
  	return textsearch_register(&bm_ops);
  }
  
  static void __exit exit_bm(void)
  {
  	textsearch_unregister(&bm_ops);
  }
  
  MODULE_LICENSE("GPL");
  
  module_init(init_bm);
  module_exit(exit_bm);